In a color television (TV) system (such as NTSC), the luminance and chrominance components ("luma" and "chroma", respectively) of a composite color television signal are disposed within the video frequency spectrum in a frequency interleaved relation, with the luma components at integral multiples of the horizontal line scanning frequency and the chroma components at odd multiples of one-half the line scanning frequency. In the NTSC system, the upper portion (i.e., about 2.1 to 4.2 MHz) of the video frequency spectrum (0 to 4.2 MHz) is shared by chroma components and high frequency luma components. The lower portion (below about 2.1 MHz) of the video frequency spectrum is occupied solely by luma components. Various comb filter arrangements for separating the frequency interleaved luma and chroma components of the video signal are known, for example, U.S. Pat. Nos. 4,143,397 (Holmes) and 4,096,516 (Pritchard).
Comb filters operate on the premise that the composite video signal from horizontal-line-to-horizontal-line or field-to-field or frame-to-frame is highly correlated. When this assumption fails, as it frequently does with program video, certain anomolies occur in the reproduced images. The anomolies result from imperfect cancellation of chroma in the luma output, and vice versa. For example, if there is an abrupt change in the amplitude of chroma between adjacent lines, scintillating serrations will occur along the horizontal edges displayed in the image for a line comb filtered (hereinafter "combed") signal. These serrations (called "hanging dots") are due to incompletely cancelled chroma in the luma channel. Alternatively, if there is an abrupt change in the luma amplitude between horizontal lines, anomalous color saturation effects will be displayed along horizontal edges. Similar undesirable effects occur in the field and frame combed signals.
Adaptive comb filters eliminate some of these artifacts caused by imperfect cancellation of chroma in the luma output, and vice versa. The adaptive comb filters typically use alternate Y/C separation functions (e.g., 1-H line comb, 2-H line comb, etc.) and, for each of these functions, calculate an index of correctness. The Y/C separation function with the best index is selected for generating the combed luma and chroma components. U.S. Pat. Nos. 4,050,084 (Rossi) and 4,636,840 (McNeely et al.) disclose illustrative Y/C separation systems having adaptive features.
In the Rossi's adaptive Y/C separation apparatus, the incoming (or undelayed) composite video signal B is bandpass filtered (hereinafter "bandpassed") to pass a band of frequencies (about 2.1 to 4.2 MHz) including the interleaved chroma components and the high frequency luma components. The bandpassed incoming composite video signal B.sub.b is delayed twice by two, 1-H delay lines to develop a pair of bandpassed, 1-H delayed and 2-H delayed composite video signals M.sub.b and T.sub.b respectively. The capital letters "B", "M" and "T", respectively, stand for the bottom, middle and top horizontal lines of the video signal, and the subscript "b" represents a bandpassed signal.
Rossi uses a plurality of comparators to compare the three bandpassed comosite video signals B.sub.b, M.sub.b and T.sub.b (i.e., undelayed, 1-H and 2-H delayed signals, respectively). Depending upon the outcome of the comparison, one of several Y/C separation functions (e.g., 1-H comb, 2-H comb, etc.) is selected to the exclusion of others to develop a bandpassed chroma signal C.sub.b.
The incoming composite video signal B is applied to a third 1-H delay line to develop a 1-H delayed (non-bandpassed) composite video signal M. The bandpassed chroma signal C.sub.b is subtracted from the 1-H delayed (non-bandpassed) composite video signal M to generate the luma signal Y, having an uncombed low frequency portion (below 2.1 MHz) and a combined high frequency portion (between 2.1 and 4.2 MHz).
One of the limitations of Rossi is that he employs arbitrary reference values, which makes the effectiveness of his apparatus variable with signal conditions. Another limitation of Rossi is that his decision process compares single samples from each of the lines (to decide which Y/C separation function is best), and is, thus, susceptible to errors due to noise.
Still another limitation is that Rossi uses three (3) 1-H delay lines for generating the four signals (B.sub.b, M.sub.b and T.sub.b and M) needed to develop the separated luma and chroma components Y and C.sub.b. Delay lines are relatively expensive to implement, both in analog and digital domains (but, particularly, in the digital domain). A further limitation of Rossi is that he uses an "all-or-nothing" selection process. (One of the Y/C separation functions is used to the exclusion of all others.)
The adaptive Y/C separation apparatus disclosed in U.S. Pat. No. 4,636,840 (McNeely et al.) overcomes some of the aforesaid limitations of Rossi. Pairs of composite video signals are developed, which are delayed from each other by an integral number of horizontal line periods (e.g., 1H, 263H, 525H, etc.). Each pair of video signals is used to generate separated luma and chroma components Y and C, and an index ("sum of cross differences") that measures the correctness of the accompanying Y and C components. These measures of correctness are compared. Depending upon the results of comparison, the Y and C components associated with the best measure of correctness are selected for further processing in the TV receiver.
McNeely et al. do not rely on arbitrary reference values, thereby providing a more accurate Y/C separation. Furthermore, McNeely et al. use a pair of samples (instead of a single sample) from each of the lines for calculating cross differences, thereby increasing noise immunity of the selection process. However, McNeely et al. use on "all-or-nothing" selection process similar to Rossi.
Another limitation of McNeely et al. is that their separated luma component is subjected to a "combing" effect over its entire band (0 to 4.2 MHz). The combing action over the high frequency band portion of the video frequency spectrum (which is shared with chroma components) has the desired effect of deleting chroma components from the luma output. Extension of this combing action into the low frequency band portion (which is not shared with the chrominance signal components), however, is not needed to effect the desired removal of chroma components, and serves only to unnecessarily delete luma components. Components in the lower end (e.g., below about 1 MHz) of the unshared band which are subject to such deletion are representative of "vertical detail" luma information. Preservation of such vertical detail is desirable to avoid loss of vertical resolution in the luma content of the displayed image.